1. Technical Field
Exemplary embodiments are related to impairment compensation, such as distortion, dispersion, non-linear effects and the like in fiber optic communication systems, and particularly to electronic phase conjugation for impairment compensation of fiber optic links.
2. Brief Description of the Related Art
To meet the ever growing capacity demand in a core network, future high-speed optical transport will likely employ fiber transmission systems with very high data-rate channels, such as 400 gigabits per second (Gb/s), 1 terabits per second (Tb/s), and the like, and high spectral efficiency (SE). With the aim of increasing SE, high-order modulation formats may be used together with tightly spaced wavelength-division multiplexing (WDM). Since advanced modulation formats typically require increased signal-to-noise ratio (SNR), performance of high SE transmission systems may be limited by fiber nonlinearity in the form of, for example, intra- and inter-channel effects. Consequently, techniques for the mitigation and/or compensation of nonlinear effects in optical transmission are attracting significant interest.
Recently, techniques based on digital backward propagation (DBP) have been proposed for compensation of fiber impairments. DBP requires a high degree of complexity. DBP is universal, system independent, and does not involve in-line symmetry. However, DPB demands very high computation resources, especially for the compensation of inter-channel effects.
Optical phase conjugation (OPC) has also been also proposed for the comprehensive compensation of fiber impairments. Using OPC, distortions are typically cancelled out when group delay and nonlinear phase-shift occur in a symmetric fashion with respect to the location at which OPC is performed. OPC is fast and enables large bandwidth and real-time compensation. However, OPC typically requires complex optical hardware and mid-link symmetry to be effective.
In OPC, a spectral inversion of an optical signal is typically performed at a midpoint of a fiber optic link to invert dispersion and nonlinear distortions experienced by the optical signal and to compensate for dispersion and nonlinear distortions that the optical signal is expected to experience in the second half of the fiber optic link. The spectral inversion implemented using optical phase conjugation is conventionally achieved using nonlinear effects, such as four-wave mixing (FWM) or second harmonic generation (SHG). To be efficient, such effects typically require highly nonlinear media, high optical power or a combination of both.
FWM has been implemented using silica to achieve optical phase conjugation. Since silica has a relatively low nonlinearity, high pump power values are required to achieve efficient wavelength conversion. This increases the efficiency of undesirable scattering effects that eventually limit the OPC performance. In addition, FWM requires phase matching which depends strongly on chromatic dispersion. One way to decrease the pump power is to increase the interaction distance. However, this makes integration difficult and aggravates problems such as dispersion fluctuations.
Semiconductor optical amplifiers (SOA) have also been used as nonlinear medium for optical phase conjugation. In contrast to silica fiber, SOAs are an active medium with high nonlinearity. SOAs can achieve large conversion efficiencies in relatively short distances which presents a compact solution compared to fiber devices. However, SOAs typically require electrical energy which creates heat and long term damage.
Together with FWM, SHG can be used to create phase conjugation. For example, periodically-poled lithium-niobate (PPLN) has been used as a nonlinear medium. PPLN can produce quasi-phase matched SHG with a high efficiency conversion. However, PPLN requires high temperatures for optimum operation which prevents integration with other components.
Both DPB and OPC are capable of compensating inter-channel nonlinear effects in WDM systems. OPC is typically restricted to point-to-point links, where all the channels share the same transmitter and receiver locations. In fiber networks with added/drop multiplexing throughout the link, compensation of inter-channel effects is typically not possible with OPC. In current optical networks, reconfigurable optical add-drop multiplexers (ROADMs) are being deployed to route information in the optical layer, which can inhibit compensation of inter-channel effects using OPC because the interacting channels do not share the same path.
Future optical networks are expected to support reconfigurable optical add/drop multiplexer (ROADMs), as well as channels operating at different baud rates and different SNRs to accommodate varying capacity demands. One scenario of interest is the transmission of a high-rate (high SNR) express channel together with neighboring lower speed (lower SNR) add/drop channels. These express channels can be limited in performance by fiber nonlinearity prior to reaching the add/drop networking channels.